It is known that dissolved substances can be separated from their solvents by use of selective membranes. For example, of great practical interest is the removal of salt from water by reverse osmosis. The efficiency and economy of such removal is of tremendous economic significance in order to provide potable water from brackish or sea water for household or agricultural use. A critical factor in desalination is the performance of the membrane in terms of salt rejection, i.e., the reduction in salt concentration across the membrane, and flux, i.e., the flow rate across the membrane. For practical applications, the flux should be on the order of greater than about 10 gallons/ft.sup.2 -day (gfd) at a pressure of about 55 atmospheres for sea water and about 15 gfd at a pressure of about 15 atmospheres for brackish water. The continuing goal of research and development in this area is to develop membranes having increased flux and/or salt rejection which are useful in desalination.
Among the known membranes used in desalination are included a large number of various types of polyamides which are prepared by a variety of methods.
Of particular interest within this broad group of polyamide membranes are crosslinked aromatic polyamide membranes. The crosslinked aromatic polyamide membranes include, for example, those disclosed in the following U.S. Patents.
U.S. Pat. No. 3,904,519, issued to McKinney et al, discloses reverse osmosis membranes of improved flux prepared by crosslinking aromatic polyamide membranes using crosslinking agents and/or irradiation. The polyamides are prepared, for example, by the interfacial polymerization of amine groups and carboxyl groups followed by crosslinking.
U.S. Pat. No. 3,996,318, issued to van Heuven, teaches the production of aromatic polyamide membranes, wherein crosslinking is achieved using a reactant having a functionality of three or greater.
U.S. Pat. No. 4,277,344, issued to Cadotte, describes a reverse osmosis membrane which is the interfacial reaction product of an aromatic polyamine having at least two primary amine substituents with an aromatic acyl halide having at least three acyl halide substituents. The preferred membrane is made of a poly(phenylenediamine trimesamide) film on a porous polysulfone support.
U.S. Pat. No. 4,529,646, issued to Sundet, shows a similar membrane in which all or a portion of the trifunctional aromatic acyl halide is replaced by cyclohexane--1,3,5-tricarbonyl chloride. Similar membranes are disclosed in U.S. Pat. Nos. 4,520,044, 4,544,484 and 4,626,468, each issued to Sundet.
U.S. Pat. No. 4,661,254, issued to Zupanic et al, discloses a reverse osmosis composite membrane formed by the interfacial polymerization of a triaryl triamine with an aromatic carboxylic acid chloride.
While some of the above referenced membranes are commercially useable, the goal of the industry continues to be to develop membranes that have better flux and salt rejection characteristics in order to reduce costs and increase efficiency of operation.